U.S. Pat. No. 4,383,229 shows clipping only of specific harmonics, which is different from the method disclosed herein.
Using clipping of the peaks of a voice signal followed by filtering in a bandpass filter has long been known. U.S. Pat. No. 3,297,882 shows such an arrangement. The latter patent shows only clipping and pass band filtering. Clipping and band-filtering alone however has never gained widespread use as means for band reduction due to the fact that clipping and band filtering according to the known art has resulted in poor voice signal quality and loss of speaker recognition.
Voice band reduction clearly is a much desired object, since it makes possible an increased number of voice channels within a frequency band of a given width. This is especially true in radio communication, including satellite communication, and carrier-based land lines wherein the available bandwidth is constrained by the availability of usable frequency bands.
Clipping of a voiceband signal according to the instant invention leads to increased voice energy levels especially above 1000 Hz where normal speech energy levels tend to roll off.
Unlike the prior art, in the instant invention clipping is applied on a broad band basis and the harmonics created in the clipping process are eliminated by means of suitable filtering as disclosed in more detail in the following disclosure.
It is therefore a primary object of the instant invention to provide apparatus and a method of frequency band reduction of a voice signal having a given original bandwidth without significant loss of voice signal quality and speaker recognition.